Under the leadership of Dr. John Godleski MD, the Particles Core brings together basic, clinical, and public health scientist to work in concert to make substantial progress in our understanding of human health effects that result from ambient particle exposure and the biological mechanisms responsible for these effects. For example, investigators of this core were among the first to epidemiologically define cardiovascular deaths as very important outcomes related to increases in ambient air pollution (Dockery, Pope et al. 1993;Schwartz 1999). In population studies, Center investigators and others identified short term outcomes, such as heart rate variability and arrhythmias that served as potential markers that could lead to cardiovascular deaths and were associated with exposure to increases in ambient particles (Pope, Verrier et al. 1999;Gold, Litonjua et al. 2000). These findings were defined in human populations, technologies were developed to use real particulate ambient air pollution in laboratory settings to study toxicological mechanisms (Sioutas, Koutrakis et al. 1995; Godleski, Vernier et al. 2000), and laboratory models with both large and small animals were developed for mechanistic studies (Clarke, Catalano et al. 1999;Godleski, Verrier et al. 2000;Gurgueira, Lawrence et al. 2002;Wellenius, Saldiva et al. 2002;Wellenius, Coull et al. 2003). In the past three years, interactions between basic scientist and epidemiological investigators fostered by this NIEHS center have substantially accelerated this area of research. The iterative process has resulted in a clearer understanding of who is susceptible, and by what mechanisms do the responses result in adverse health outcomes. Most importantly, we have begun to reach a level of understanding in which multiple diverse outcomes are mechanistically linked.